Fix more misspelled comments and strings.

src/liballoc/arc.rs

@@ -184,7 +184,7 @@ impl<T: Share + Send> Drop for Arc<T> {
 
         // This fence is needed to prevent reordering of use of the data and
         // deletion of the data. Because it is marked `Release`, the
-        // decreasing of the reference count sychronizes with this `Acquire`
+        // decreasing of the reference count synchronizes with this `Acquire`
         // fence. This means that use of the data happens before decreasing
         // the refernce count, which happens before this fence, which
         // happens before the deletion of the data.